The present invention relates to a thermostatic switch for monitoring electrically heated devices, the switch being of the type including an insulating substrate on which one end of a contact spring is fastened, with the free end of the spring, being provided with a contact which cooperates with a countercontact likewise fixed on the insulating body, the switch further including electrical leads and a bimetal disc which is in operative connection with the contact spring so that if a predetermined temperature is reached, the contacts are opened or closed.
German Offenlegungsschrift [Laid-Open Application] No. 2,821,457.5 discloses a switch in which one side of a contact spring is connected to an insulating body with the free end of the spring being provided with a contact that cooperates with a countercontact fixed to the insulating body. One or a plurality of bimetal discs are in operative connection with this contact spring and the bimetal discs are each provided with a hole in its center to form a concentric mount on the contact spring.
A drawback of this design is the limited service life of the bimetal discs since the number of possible switchings is reduced by the effect of the center hole. With repeated switchings there exists the danger that cracks develop in the bimetal disc starting at the center hole, reducing the original tensioned state of the disc so that proper snapping is no longer assured. This would have an effect on the switching behavior of the contacts, i.e. with such bimetal discs momentary opening and closing of the contacts would no longer be possible.
To overcome this drawback, German Offenlegungsschrift No. 2,904,341, which is for a Patent of Addition to the above-cited application, proposes a design for the contact spring such that bimetal discs can be arranged at the contact spring without having a center hole. The contact spring here is given a stamped curvature in its center and resilient flaps at its two longitudinal sides which have one end fixed to the contact spring and are angled at their other, free ends. Both resilient flaps are slightly prebent in the same direction as the stamped-in curvature in the center of the contact spring.
The bimetal disc is held, on the one hand, by L-shaped flaps extending from the contact spring and, on the other hand, by the resilient flaps disposed at its longitudinal sides.
A particular drawback of this design of the contact spring has been found to be the fact that if an external mechanical force acts on the two resilient flaps, for example during transport or installation of the temperature switch in an electrical device, the bimetal disc is laterally displaced out of its mount and thus comes loose from the contact spring.
A further drawback of the resilient flaps is the different contact pressure occurring at the contact spring as a result of nonuniform angulation or subsequent bending of the flaps.